[virtio] Fix queue size calculations

The queue size calculations currently do not take into account the
fact that each packet requires a pair of descriptors.  If the device
happens to present an extremely small queue (16 descriptors for Q0, 32
descriptors for Q1) then this will result in the driver submitting
descriptors beyond the queue's descriptor count.  This does not result
in any invalid memory accesses (since the descriptor ring length is
rounded up to a 4kB boundary), but does result in an unusable network
device.

Fix by scaling the packet count and descriptor count values as
required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>